Abstract [en]

The compound energy formalism, CEF, involves many model parameters. They are evaluated to give the best fit to the experimental information. The optimisation is simpler if less parameters need to be adjusted.The maximum number of independent parameters that can be evaluated depends on the information available. The best choice of parameters is first discussed for simple ionic substances with an internal variable, then for solutions of two or four such substances.

To reduce the number of parameters, independent parameters are conveniently defined as combinations of primary model parameters. That may be possible when there is an internal variable,which can take only one value, the value that minimizes the Gibbs energy. Such combinations may be regarded as the true optimisation parameters and they may be used actively during an optimisation. The present discussion deals with substances with an internal variable and mixtures, which may have more than one internal variable.

The conclusions apply equally well to non-ionic systems if the information is limited to stoichiometric compositions. The optimisation parameters should then be defined for stoichiometric overall compositions.

In thesis

Kjellqvist, Lina

KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.

2009 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

The Fe-C-Cr-Mn-Ni-O system is of fundamental importance when describing the influence of oxygen on high alloyed steels. Both solid and liquid phases are of great interest: The solid phases regarding oxidation processes like the formation of oxide layers, inner oxidation, sintering processes and high temperature corrosion. The liquid phase is of interest concerning the interaction between steel and its slag in a metallurgical context. In this thesis the thermodynamic properties of this system is described using the Calphad technique. The main idea of the Calphad technique is to describe the Gibbs energy of all phases in the system as a function of temperature, pressure and composition using appropriate thermodynamic models. When thermodynamic descriptions of all phases taking part in the system are modelled and described in a database, the equilibrium state could be calculated with a software that minimizes the total Gibbs energy.

Models within the compound energy formalism are used for all solution phases, among them the ionic two-sublattice liquid model, to describe both the metallic and oxide melts. All simple spinels (Cr3O4, FeCr2O4, Fe3O4, FeMn2O4, Mn3O4, MnCr2O4, NiCr2O4, NiFe2O4, NiMn2O4) within this system are described using a four-sublattice model. In this thesis several binary and ternary systems have been assessed or partly reassessed. The Fe-C-Cr-Mn-Ni-O database achieved can be used with an appropriate thermodynamic software to calculate thermodynamic properties, equilibrium states and phase diagrams. In general, the agreement between calculated and experimental values is good.